1. Field of the Invention
The present invention relates to improvement in root canal meters for diagnosis and treatment in dental practice.
2. Description of Prior Art
As a meter for measuring root canal length electrically there have been known those in which resistance is measured between a measuring electrode and an oral electrode connected to an oral soft tissue (See, for example, Japanese Patent Publication No. 25381 of 1987.) and in which the impedance between both electrodes is measured (See, for example, Japanese Patent Publication No. 2817 of 1987.).
Of the aforementioned patent publication, the former is based on detection of a decrease in the resistance as the tip of the measuring electrode approaches the apical position, while the latter is based on detection of a decrease in the impedance under the same condition. Since the measuring electrode and the oral electrode are deemed to be an equivalent circuit in which a resistor and a capacitor are connected in parallel, the latter, from the viewpoint of measuring principle, is considered better fit for the actual situation.
The rate of change of resistance and capacitor capacity in the aforementioned equivalent circuit when the tip of the measuring electrode is at the dental neck section at the center of the root canal and when it has reached the apical position at the tip of the root canal, is substantially larger in capacitance than resistance, especially the impedance is subject to significantly marked change in the vicinity of the apical position. Hence measured data detected in the form of current or voltage remain low when the measuring electrode is apart from the apical position without any significant increase and starts increasing sharply when the apical position is approached. FIG. 3 shows an example of such a situation, in which the abscissa shows the distance to the apical position and the ordinate the measured data.
Hence, if such measured data are displayed as they are, the displayed value, which remains small with no marked increase when the tip of the measuring electrode is apart from the apical position, suddenly increases when it has come to be approximately 1 mm off the apical position, this making it extremely difficult to use. Such tendency is noticeable to some extent even with that of the resistance detection system but is particularly significant with that of the system of detecting change in impedance.